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Day 115 - First Shadow Flights

Early morning fog on the way to the launch
site.

First needed to do a bit of house keeping at
the launch site.

Prepping rocket for maiden flight.

Compulsory pose before launch.

Arming electronics and starting video and
altimeter.

LCO ready at his post. Rocket has 10psi to
check for leaks.

Launched at 330psi.

(Photos: Andrew
Eltobaji)

Short contrail soon after the end of the air
pulse.

View back to launch control.

Just starting the air pulse.

New industrial development.

Apogee photo from 947 feet (288m)

Looking in the other direction at the ever
expanding housing estate.

Looking at the launch site on the way down.

Looking south west

"Shadow's shadow"

Self portrait shortly
before landing.

Panorama composed of images on the way down.

Descending at 6.1m/s

Rocket lands close by.

Grass landings are always a bonus.

Downloading altimeter data and video.

Setting up for second launch.

Arming electronics.

Almost at target pressure.

Launched at 360psi.

(Photos: Chris
Barnes)

At least we don't have far to walk. The
rocket missed the ladder by mere centimeters.

Carefully removing the aft end of the
rocket.

Rocket penetrated about 1 foot into the soft
ground.

Digging out the remnants of the nosecone and
payload bay.

"John, is this is how you play Minecraft?"

Discovering that the altimeter is still
recording.

Deployment mechanism. STII and servo
survived, but the uMAD is dead.

Top of rocket body that protected the
altimeter.

Altimeter and top bulkhead mostly survived.
HD cam is destroyed.

Debris remains from nosecone and upper
payload bay. We will reuse the nosecone tip.

Connecting up the STII and servo to the
battery - all still worked fine!

Nozzle and bulkhead half way down rocket.
This will be pushed back in place.

After the delays over Christmas and rainy
weather we finally got a break on a
beautiful morning with ideal launch
conditions. On the way out to Doonside we
drove through heavy fog, but by the time we
got on location the fog had lifted. We
arrived about an hour earlier than normal to
help out as NSWRA was having a working bee
to get the tall grass mowed down around the
launch area.

Rocket preparation went well. We chose
the smaller of the two parachutes we had
prepared (820mm diameter). The rocket was filled with 1.3L of
water and set up on the launcher.
We had the extra long hose extension this
time and the direct scuba tank connection
with the large pressure gauge. For the
higher pressures we fill the rocket straight
from the tank and shut it off when it gets
to the desired pressure. We took just over 2
minutes to pressurise the rocket to keep
compression heating down.

The rocket also used the break wire
option to trigger the backup timer just in
case the high acceleration damaged the built-in 2G acceleration switch.

Launch 1

The first launch was at 330psi
(~23 bar) since the pressure chamber had
only been hydro tested to 335psi. The rocket
released easily and made a loud bang during
the air pulse. (You just don't hear it well
on the video) The rocket went straight up
and you could see it coast for a long time.
Right at apogee the parachute popped out and
the rocket slowly drifted down and landed in the tall grass maybe
70m away from the pad.

The maximum altitude reached
was 947 feet (288 m) with a total
flight time of 54.2 seconds. This was our
new personal best altitude.

In the onboard video you can hear the uMAD continue to beep it's armed state even
after the parachute deployed. This tells us
that the parachute was ejected by the backup
timer rather than the uMAD. In the video
you can see that when the parachute popped,
the rocket was still mostly pointed upward
and since the parachute is attached at the
top of the rocket, the uMAD never had the
chance to fire.

From the altimeter data we can roughly
calculate that the peak velocity on the way
up was: ~260 feet/s or 79m/s
or 285km/h or 177mph

We also see from the altimeter plot that
the descent rate for this parachute was: 20
feet/s or 6.1m/s

Video from the on-board camera showed
that the rocket rolled approximately 270
degrees from launch to apogee. This means
that the fin alignment jig worked quite
well. With the sunny conditions the video
turned out great.

With all the humidity in the air the slow mo video showed
that the rocket produced a brief contrail
about 20 meters above ground which was
several meters long. This was after the end
of the air pulse when very cold air was
coming out of the rocket. (see picture on
left) We saw this on both flights, but have
never seen it on our regular rockets.

Launch 2

Seeing that we were close to the 1000'
mark we decided to go for a higher pressure
of 360psi (~25 bar) for the second
flight. We were going to have to pressure
test the rocket on the pad to see if it
would hold. We were reasonably confident
since tests on short lengths of the tube
held at least 500psi.

Rocket prep and setup were identical to
the first launch.

When Paul tried to launch the rocket he
had a hard time getting it to release. I'm
not sure why that was because the pressure
was only an extra 10% and shouldn't have
been an issue. But after a couple of tugs
the rocket launched in a similar manner to
the first launch. Straight up .... and
straight down again. The rocket pitched over
at apogee but no parachute came out. The
rocket came down very fast and you could
hear it whistle shortly before it hit. It landed a couple of
meters from the launch pad!

Because the soaked soil was very soft it
buried itself about 1 foot into the ground. The entire nosecone section and
payload frame
were destroyed. We had to use the old shovel
recovery technique for this one.

The altimeter and camera were at the
lowest point in the payload and so had the
maximum protection. Sadly though, the camera
didn't survive and the SD card was bent and
broken. We tried to read the data off it,
but it just heated up when we plugged it
into the adaptor. To our surprise the
batteries survived and were still powering
the altimeter and the altimeter was still
recording! The altimeter had a damaged LCD
display from a failure a couple of years
back, and this crash, just did more damage
to it. The rest of the altimeter is mostly
in tact. The computer was able to connect
and download the data without issues though.
We will most likely retire it now, or
use it on other non critical flights.

The data showed 1017 feet! (310
m) . Woo hoo :) This is now our new
personal best altitude though if this had
been a certification flight, it wouldn't
have passed due to the damage, so we are
going to have to try it again. The data
plot also had a very large negative spike on
impact. This would have been caused by the
pressure increase in the payload bay as it
collapsed around
the altimeter.

The peak velocity was 275 feet/s
or 83.8 m/s or 302 km/h or 187mph

Here is a highlights video from the two
launches:

What went wrong?

We still don't know exactly but have
some ideas. We did not re-test
the deploy mechanism after the first flight,
as it had worked fine, and there was no
reason to believe that something may have
been damaged. It is possible that something
went wrong when the rocket landed, but when
we loaded the parachute for the second
flight, the servo had moved to the locked
position, the piston was also okay to lock
down.

After the crash we noticed that the
ground wire was cut between the batteries
and the deployment mechanism. At first we
thought this may have somehow happened when
we re-assembled the rocket, but the uMAD was
beeping on the pad which means it had power
so the line wasn't cut at that point. The
altimeter is connected to the batteries
through a different set of wires and those
were fine.

After pulling apart the ball of tangled
aluminium, balsa and fiberglass that once
resembled a deployment mechanism, we
discovered the Servo Timer II was undamaged
except a couple of bent pins. Even the RC
servo still worked! Though the servo motor horn was broken
it was in its deploy
position meaning that it was sent a
signal to move and it did. The linkage between the
servo horn and the lever arm was also
intact. If the servo horn was in the deploy
position then the release arm must have also
been pulled back. We were able to reconnect
the batteries and both the STII and servo
functioned as they were supposed to.

As a result we can only conclude that there
had to be a mechanical failure of the
piston due to the high G forces. (around 55Gs )

We had tested the piston
dozens of times on the ground and we never
had a stuck piston issue. Our best guess at what might have happened
is that during acceleration a portion of the
parachute, or shock cord may have wedged
itself in the gap between the piston and the
payload bay wall and prevented it from
sliding out. The piston appears to have
never moved as the springs were still
compressed as they are when in the stowed
position.

It is also possible that somehow the
nosecone was wedged so tightly into the body
tube during acceleration that the springs
weren't strong enough to push the parachute
and nosecone out. The springs exert about
50N of force to eject the parachute.

The camera and SD card were damaged and
will need to be replaced. We have a spare
camera so we will use that. The altimeter is
usable, but we will most likely replace it.
The uMAD was destroyed.
The nozzle bulkhead flew half way down the
rocket (up to the coupler) on impact. This
bulkhead is just free floating anyway so we
will just push it back into the tail and it
should be good to go. Because the bulkhead
flew forward during impact it would have
reduced the shock somewhat as well. The
forward bulkhead also survived without
damage.

As John was digging out the nosecone
debris, he hands me two bits of wire and
says "Dad, here you can still use these" :)

The top end of the pressure chamber will be
cut square and extended again with a
coupler. We could potentially increase the
length of the rocket somewhat during the
repair and give the rocket a little more
capacity. This would help offset the added
weight of the repair coupler. We will also
try to make the payload bay and ejection
mechanism a little shorter and lighter.

Though this will be a bit more of an
expensive fix, the offering to the rocket
gods was well worth the milestone. We are
looking to make the next version of the
deployment system fully redundant with two
isolated circuits and separate batteries.

What went right

Despite the crash there were a lot of
good things that came out of the 2 flights.

The rocket did not bend (at least
what we could see) during the launch and
was able to withstand the acceleration
forces. This was one of our major
concerns as that was a major problem
with our FTC rockets.

The launcher worked well to hold and
seal the rocket.

The rocket came off the launch tube
cleanly.

The deployment mechanism worked ....
at least once :)

The parachute attachment point
worked well.

The rocket didn't spin very much.

The nosecone did not separate on
burnout - This was another one of our
main worries because it had to be fairly
loose so that the springs could push it
out. The parachute being fairly loose in
the payload section meant that it would
fly upwards after burnout also pushing
against the nosecone.

Pressure chamber held up well under
the pressures.

The rocket was able to land without
damaging the fins or nozzle (on both
occasions! :) )

The parachute was a good size for
this rocket, and did not tear or rip out
the shroud lines.

Simulating the flights

During development we used
Dean Wheeler's simulator for predicting
the rocket's performance. With the best
estimates for the various rocket parameters
we had a predicted altitude of: 980 feet
(299m) and a max speed of 79m/s for the first launch
at 330psi.

The results from the first flight put the
simulator about 30 feet over. The predicted peak
acceleration was close to 50G at burnout.

The simulation for the second flight @360psi
predicted: 1099' (335m) and a max speed of
85 m/s ( 306km/h or 190 mph). Here the
simulator was 82 feet over the measured
value.

Leaving all other parameters the same, we
tweaked the drag coefficient on the
simulator so that the predicted altitude
matched the first flight's measured altitude.

Re-running the simulator with the new
drag coefficient for the second launch we
get a predicted altitude of 1036feet(316m)
which is close enough. So from now on we can
use the new drag coefficient for the rocket
and hopefully get more accurate predictions
for the next round of flights.

The simulator also predicted a crash down
velocity of about ~250km/h ( 155mph )!